1. Field of the Invention
The present invention relates to a lightweight structural material of a reinforced, foamed resin, and to a process for preparing the same. More particularly, the present invention relates to a structural material constructed of two surface layers each of which comprises fiber reinforced foamed resin, and a core layer provided therebetween which combines or merges with the surface layers to form a single body, and to a process for preparing such an article.
2. Description of the Prior Art
Hitherto, various shaped articles of foamed resins have been proposed. For instance, there have been known foamed resins reinforced by covering the surface thereof with asbestos slates or metal plates, or by dispersing fibers thereinto. However, when asbestos slates are employed, the articles are weak in impact strength and exhibit poor workability, for example, when they are cut with a saw. On the other hand, in the case of metal plates the articles also have poor workability. In addition, a sufficiently lightweight cannot be attained in either case. When the surface of a foamed resin is covered with a synthetic resin plate reinforced with fibers, the impact resistance of the article is not sufficiently high because of the rigid surface. Further, satisfactorily high compression strength and good workability also cannot be attained.
In addition to the above-described foamed resin materials, plate-like shaped articles having a sandwich construction are known in which for example fiber reinforced resin plates (abbreviated as FRP) are used as a surface material and honeycomb materials are used as a core material. These articles show poor workability on sawing, planing and nail-driving because of the rigidness of the FRP employed as a surface material. It is difficult to render these articles lightweight because, while the core materials contained therein are lightweight, the combined use of the FRP increases their respective specific gravities. These articles are also weak in impact strength. Articles utilizing foamed resins as core materials suffer from the defect that they are easily deformed by compressive stress, and the articles suffer from the defect that they are weak to certain stresses, especially bending stress because the junction between the surface material and the core material is small.
On the other hand, foamed resins reinforced by dispersing fibers therein are excellent in impact resistance, workability and bending and compression strength. One example of a foamed resin fiber reinforced material is described in U.S. Pat. No. 4,025,257 and Japanese patent application (OPI) No. 107360/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, as the foaming magnification increases, the foams communicate with one another at the interfaces of the resin foams with fibers contained therein and water absorption tends to occur. Consequently, an increase in weight and a decrease in heat insulating ability due to water absorption tend to take place and a problem arises. Though it is necessary to limit the foaming magnification to a low level under these circumstances, low magnification is undesirable from the standpoint of providing lightweight and economical articles.
U.S. Pat. No. 3,917,774 discloses a continuous process and apparatus for preparing an elongated foamed resin article reinforced by continuous fibers in which continuous fibers are advanced in parallel relationship in a sheet-like form and are impregnated with a liquid composition capable of forming a foamed thermoset resin. The impregnated fibers are then passed through a gathering means which gathers the fibers into a bundle having a cross-section approximating that of the desired article thereby uniformly dispersing the liquid composition in the fibers. The bundle of fibers is then advanced through a movable molding passage of three or more endless belts arranged parallel to each other so as to form a passageway which has a cross-section perpendicular to the direction of advancement of the bundle of fibers corresponding to the desired cross-section of the article. The belts contact and move with the impregnated bundle of fibers.